A transformer typically includes two or more inductively coupled windings that effect the transfer of electric energy from one circuit to another with a change in voltage, current, phase, or other electric characteristic. Transformers are used in many different electrical devices. For example, transformers are used in modern circuit breaker devices for sensing current in an electrical distribution circuit and providing a signal indicative of the sensed current to electronic circuitry, known as a trip unit, housed in the circuit breaker.
Conventional circuit breaker devices with electronic trip units typically include a current transformer (xe2x80x9cCTxe2x80x9d) disposed around a line conductor of a distribution system providing electrical power to a load. The current transformer has a multi-turn secondary winding electrically connected to the circuit breaker""s electronic trip unit. The secondary winding is used to sense a current overload or imbalance in the aforesaid line conductors and, in response thereto, provide an output signal proportional to the current overload or imbalance to the trip unit. Upon receipt of such a signal the trip unit initiates an interruption of the current supplied to the load through the line conductors. The secondary winding may also be used to provide operating power to the electronic components within the circuit breaker""s electronic trip unit.
Operationally, the load current in a circuit breaker can cover a very wide range. Unfortunately, the magnetic materials commonly available for the core of the current transformer limit the dynamic range of the sensing device. Peak flux density is a limiting factor at the upper end of the dynamic range, while core loss/declining permeability is a limit at the lower end. For a given core material and required accuracy, these parameters limit the operating range of the current transformer. While the dynamic range could be extended by increasing the volume of the core material and/or the turns of a secondary winding, these solutions increase the size of the current transformer, which is often critical.
Circuit breakers are designed to conform to published time-current curves to an accuracy of about +/xe2x88x9220 percent, which may apply to either current or time. In general, a circuit breaker is specified not to trip at its rated current value, and must trip at a current of perhaps 150 percent of its rating. At higher currents, breakers are expected to trip instantaneously, or with no intentional delay, which is typically no greater than 0.02 seconds, or about one cycle.
Often, a toroidal current transformer having a core in the shape of a toroid is utilized. A continuous, toroidal core provides a desirable, full dynamic range. However, the use of this type of core in a current transformer for use with a trip unit is not ideal. A trip unit is required to power-up and trip on the first half cycle. Therefore, it is necessary for the current output by the current transformer to have a uniform-sized first half cycle. In other words, it is necessary to determine the current output within the first half cycle, rather than waiting for a first full cycle to determine whether the breaker is in a trip condition. However, due to the initial flux density in the core being zero at power up because the field is zero, the current transformer output is about half of what it is after about the first cycle. While presently employed methods provide for delaying the determination of a trip condition until the second half cycle, such a delay is undesirable and may cause significant damage to the circuit breaker and/or circuit.
Another commonly used method to compensate for the nonuniform first half cycle output includes having an electronic trip unit (xe2x80x9cETUxe2x80x9d) trip the circuit breaker if the ETU senses a current in the first half cycle that is twice as large as normally needed to trip in the steady state operation. This is based on the assumption that the initial condition in the CT magnetic flux will limit the output on the first half cycle only. In other words, during steady state operation, the magnetic field changes from a positive maximum to a negative maximum in a typical output current having a typical sine wave form. However, on the first half cycle, the field starts at zero when the circuit breaker is powered up. This initial magnetic variation causes a xe2x80x9cstuntedxe2x80x9d output for the first half cycle at power up. Thus, there is a need for accurate determination of a trip condition at the first half cycle, as the stunted output of the CT is not an accurate reflection of the current flow compared with the output of the CT in steady state operation.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a method of protecting a power circuit within a first cycle after powering up using an electronic trip unit and a current transformer to sense current, the method comprising: accepting a first trip threshold value at power up; sensing for a trip condition in the power circuit within about a first half cycle of the first cycle with reference to the first trip threshold value, the trip condition comprising an over-current condition; generating a trip signal if the trip condition is sensed with reference to the first trip threshold value; accepting a second trip threshold value after the first half cycle if the trip condition is not sensed; sensing for the trip condition with reference to the second trip threshold value; and generating a trip signal if the trip condition is sensed with reference to the second trip threshold value.
In an alternative embodiment, a trip unit for determining a trip condition within about a half cycle after power up using a current sensor to sense current is disclosed, the trip unit comprising: a current transformer as the current sensor to sense current; an input for inputting a signal from the current transformer, an analog to digital converter converting the signal into digital form; and a controller having first and second trip threshold values stored therein for determining an instantaneous trip condition, the controller accepting the first trip threshold value at power up, the controller accepting the second trip threshold value and replacing the first trip threshold value with the second trip threshold value after power up, the controller monitoring the signal and determining whether an over-current condition exists with reference to one of the accepted trip threshold values, the controller causing a trip signal to be output in the case of an over-current condition.